Magnetic resonance imaging (henceforth abbreviated as MRI) devices irradiate a radio frequency magnetic field of a specific frequency to a subject placed in a static magnetic field to excite nuclear magnetization of atomic nuclei such as those of hydrogen contained in the subject, and detect magnetic resonance signals generated from the subject to obtain physical and chemical information. Measurement methods using MRI devices include, besides magnetic resonance imaging in which images are formed from magnetic resonance signals, the MRS measurement in which magnetic resonance signals acquired from one to several regions are separated into groups of signals for each molecule on the basis of difference of resonance frequency (henceforth referred to as chemical shift) due to difference in chemical bonds of various molecules to acquire information of metabolites (refer to, for example, Patent document 1).
The method described in Patent document 1 is a method called PRESS method, and it is currently most frequently used as a method for localizing an objective region of spectroscopy. In this method, together with a radio frequency magnetic field pulse for exciting nuclear magnetization, a gradient magnetic field pulse for selection of a predetermined slice is applied, then, together with a radio frequency magnetic field pulse for reversing the nuclear magnetization, gradient magnetic field pulses for selecting slices of two directions perpendicular to the aforementioned slice are applied, respectively, and magnetic resonance signals generated from a region where the three slices intersect are measured. Then, the measured magnetic resonance signals are subjected to Fourier transform in the time axis direction to acquire magnetic resonance spectrum signals.
The MRS measurement has a significant advantage that metabolites existing inside a subject can be non-invasively measured, which cannot be obtained by other measurement techniques. However, since concentrations of metabolites contained in a subject are extremely low, the signal versus noise ratio (henceforth referred to as SNR) frequently becomes low. Therefore, in the MRS measurement, the measurement is generally repeated about several tens to several hundreds of times, and the results are integrated to secure the required SNR and thereby increase accuracy of the result.
Moreover, with the MRS measurement, comparative measurement may be performed for a normal region and a pathological region. For example, when the object of the measurement is the head of human body, the measurement is performed for a pathological region and a normal region at a position line-symmetric to the pathological region with respect to the longitudinal fissure of cerebrum for comparison. However, since a region to be selectively excited is localized (specified) with perpendicularly intersecting three slices in the PRESS method as described above, if even one slice is common to the sets of slices for specifying both the measurement regions, during the selective excitation of one region, the other region is also pseudo saturated. Therefore, until thermal equilibrium is restored after the measurement of a selectively excited region, measurement for the other region cannot be performed. Therefore, when the measurement is repeated to secure SNR as described above, after measurement of one region, the time for waiting for restoration of thermal equilibrium of the region cannot be used for measurement of the other region.
In order that selective excitation of one region should not affect the other region, there is proposed a technique of applying gradient magnetic fields along axes each tilted by 45° from the X, Y and Z axes, so that the three slices used for selective excitation of one region (measurement region V1) and three slices used for selective excitation of the other region (measurement region V2) do not intersect one another, as shown in FIG. 19A (refer to, for example, Non-patent document 1). According to this technique, even if the one region (measurement region V1) is selectively excited, the other region (measurement region V2) maintains the thermal equilibrium, and therefore, immediately after the measurement of the selectively excited region (measurement region V1), the measurement of the other region (measurement region V2) can be started.
Further, as another method for shortening the time required for the MRS measurement of two regions, there is a method called STEAM method. This method uses three 90° pulses as the radio frequency magnetic field for excitation, and as shown in FIG. 20, two echo signals of different radio frequency magnetic field pulse irradiation intervals called TM (TM1 and TM2) are generated from two different regions in one measurement (within 1 TR) to obtain a shorter measurement time (refer to, for example, Patent document 2).